1. Field of the Invention
The present invention relates to a channel estimation technique for wireless communications, particularly to a channel estimation technique for multi-carrier system that utilizes orthogonal frequency division multiplexing (OFDM) modulation scheme.
2. Description of the Prior Art
Wireless communication systems convert data into signals that are transmitted without wired links to guide their propagation. The signals are thus subject to multipath propagation arising from diffraction, scattering, and reflection, in addition to loss of energy. Moreover, the signal attenuation varies with time and distance, resulting in a phenomenon known as fading. Since the signal receiver must be able to recover the original data from the received signal, a communication system must have hold of the characteristics of the transmission channel's response, so that various information processing techniques can be used to compensate the signal loss in wireless channel transmission.
Due to its higher wireless bandwidth efficiency and noise tolerance, orthogonal frequency division multiplexing (OFDM), among all methods, has been adopted in many communication standards, including 802.11a/g WLAN, WiMAX, digital audio broadcasting (DAB) and digital video broadcasting (DVB). In addition, OFDM system can utilize known pilot signals to estimate the channel responses at the pilot subcarriers and thence the channel responses at other subcarriers (especially the data subcarriers). The above-said channel estimation often makes use of interpolation and extrapolation techniques, but these two types of techniques have very similar mathematical structures and people skilled in the art can easily convert interpolation techniques for extrapolation use and vice versa. For convenience and clarity, the present invention uses the term “interpolation” to indicate both interpolation and extrapolation for the following discussions, which also accords with common usage of terms in this art.
As examples of OFDM channel response estimation, both US patent publications 2008/0049598 and 2008/0008258 employ linear minimum mean-square error (LMMSE) interpolation to estimate the data subcarrier channel responses, wherein the needed channel response correlation values are calculated from existing subcarrier channel response estimates (such as the estimates at the pilot subcarriers) via cross-multiplication and averaging. On the other hand, US patent publications 2008/0144730, 2008/0137788, 2005/0105647, 2007/0110172 and 2008/0084817 also employ LMMSE interpolation for channel response estimation, but they assume that the power-delay profile (PDP) of the channel has a certain form, such as exponential, uniform, or some other shape. They compute the parameters of the PDP with specific methods, from which the corresponding channel response correlation values can be obtained and thence the interpolator coefficients for channel response estimation can be calculated.
However, the first group of techniques mentioned above requires a sufficiently large number of pilot subcarrier channel response estimates, or else the calculated correlation values will contain significant noise. Yet the more the pilot subcarrier estimates, the greater the amount of computation required in the system. The second group of techniques requires the use of specific channel PDPs for channel estimation. For example, the US patent publication 2008/0144730 mentioned above utilizes a uniform PDP with a worst-case delay spread, but the technique for estimating the delay spread is not given. The US patent publication 2008/0137788 also uses a uniform PDP to estimate the channel delay spread, but neither is the estimation technique described. The US patent publication 2005/0105647 estimates the shape of the channel impulse response first and then decides to use the uniform or the exponential PDP. However, how to make the above decision based on the shape of the response is not stated. And there is no clear description of how to calculate the parameters for the two types of PDP. Furthermore, the method requires to estimate the channel impulse response. The US patent publication 2007/0110172 applies inverse fast Fourier transform (IFFT) to the pilot subcarrier channel response estimates to acquire the channel impulse response. Based on the channel impulse response, it then calculates the parameters of an exponential PDP. However, this technique requires specific pilot subcarrier configurations that are not compatible with the signal structures of some systems, such as WiMAX. Lastly, US patent publication 2008/0084817 also uses IFFT, through which it finds a number of points in the channel impulse response that have greater strengths and uses these points to construct the correlation function of the channel responses. Unfortunately, the modeling of the channel impulse response points that have greater strengths as having independent and identical distribution (i.i.d.) when constructing the correlation function is inappropriate.
In summary, the accuracy of channel estimation has direct impact on the performance of the communication system. Conventional LMMSE channel estimation methods either require a large quantity of pilot subcarrier channel response values or require the pilot subcarriers to have some regular configuration. As a result, conventional designs exhibit deficiencies and cannot satisfactorily address the needs of wireless uplink and downlink transmissions under fast motion. The present invention teaches a novel channel estimation technique that can effectively resolve the above problems of the conventional techniques.